1,3-Dicarbonyl-2-ketimines. Hydrolysis of 1,3-dimethyl-5-(p-tolylimino)barbituric acid

Structure-reactivity coefficients for general acid catalysis of semicarbazone formation

“…Sayer and DePecol have reached a similar conclusion from the observation that the Brjánsted coefficient for the hydrolysis of l,3-dimethyl-5-(p-tolylimino)barbituric acid is essentially the same as for carbinolamine reactions of benzaldehyde derivatives that have much less electron-withdrawing substituents on the carbon atom. 22 If the a, ß', and p coefficients can be represented by the horizontal, diagonal, and vertical coordinates, respectively, on a single square diagram, such as Figure 4, or on a transformed diagram in which the ß' = y' coordinate is vertical,9 then the relationship between the pxy,pxy, and px coefficients is given9 by Pxy' = Pxy ~Px (7) This relationship is a quantitative statement of the qualitative expectation described above, that p as well as /3ig should increase if the position of the transition state shifts upward with increasing strength of the catalyzing acid (Figure 5B); however, it shows that the relationship also depends on the Hammond coefficient px (eq 4). The experimental fact that px > 0 means that pxy should be at least as large as pxy>.…”